3.12.1 Overview

This example demonstrates using the MITgcm to simulate the planetary atmospheric circulation in two ways. In both cases the simulation is configured with flat orography. In the first case shown a $2.8^{\circ} \times 2.8^{\circ}$ spherical polar horizontal grid is employed. In the second case a cube-sphere horizontal grid is used that projects a cube with face size of $32 \times 32$ onto a sphere. Five pressure corrdinate levels are used in the vertical, ranging in thickness from $100\,{\rm mb}$ at the bottom of the atmosphere to $300\,{\rm mb}$ in the middle atmosphere. The total depth of the atmosphere is $1000{\rm mb}$ . At this resolution, the configuration can be integrated forward for many years on a single processor desktop computer.

The model is forced by relaxation to a radiative equilibrium profile from Held and Suarez [26]. Initial conditions are a statically stable thermal gradient and no motion. The atmosphere in these experiments is dry and the only active ``physics'' are the terms in the Held and Suarez [26] formula. The MITgcm intermediate atmospheric physics package (see 6.10) and MITgcm high-end physics package ( see

) are turned off. Altogether, this yields the following forcing (from Held and Suarez [26]) that is applied to the fluid:

$\displaystyle \vec{{\cal F}_{u}}$	$\displaystyle =$	$\displaystyle -k_{v}(p)\vec{u}$	(3.62)
$\displaystyle {\cal F}_{\theta}$	$\displaystyle =$	$\displaystyle -k_{T}(\phi,p)[\theta-\theta_{eq}(\phi,p)]$	(3.63)

where ${\vec{\cal F}_{u}}$ , ${\cal F}_{\theta}$ , are the forcing terms in the zonal and meridional momentum and in the potential temperature equations respectively. The term $k_{v}$ in equation (3.62) applies a linear frictional drag (Rayleigh damping) that is active within the planetary boundary layer. It is defined so as to decay with height according to

$\displaystyle k_{v}$	$\displaystyle =$	$\displaystyle k_{f}{\rm max}(0,(p_{\rm {k}}/p^{0}_{s}-\sigma_{b})/(1-\sigma_{b}))$	(3.64)
$\displaystyle \sigma_{b}$	$\displaystyle =$	$\displaystyle 0.7$	(3.65)
$\displaystyle k_{f}$	$\displaystyle =$	$\displaystyle 1{\rm day}^{-1}$	(3.66)

where $p_{\rm {k}}$ is the pressure level of the cell center for level $\rm {k}$ and $p^{0}_{s}$ is the pressure at the base of the atmospheric column.